1. Field of the Invention
The present invention relates to beam scanning systems for use in an electrophotographic type image forming apparatus and, more particularly, to a beam scanning system for diffracting and deflecting beams emitted from light sources using a disc, in which the arrangement of reflecting mirrors that reflect the diffracted and deflected beam toward a photosensitive medium is improved.
2. Description of the Related Art
In general, beam scanning systems are employed by electrophotographic image forming apparatuses for use in forming an electrostatic latent image on a photosensitive medium such as a photoreceptor web by, for example, scanning beams emitted from a laser scanning unit and a light source. Recently, a multi-beam scanning system which diffracts and deflects beams emitted from light sources by adopting a rotary deflection disc, instead of by adopting a rotary polygon used in a conventional beam scanning system, has been introduced. FIG. 1 shows a schematic configuration thereof.
Referring to FIG. 1, the beam scanning system includes a light source 10 and a deflection disc 11 rotatably mounted over the light source 10. The deflection disc 11 is coupled to a driving motor 12 which rapidly rotates the deflection disc 11. The deflection disc 11 includes a plurality of sectors having diffraction patterns formed on the surface thereof.
A beam emitted from the light source 10 is diffracted by the diffraction patterns while passing through a rotating deflection disc 11. Since the diffraction patterns are formed to have different diffraction angles according to the rotation angle of the deflection disc 11, beams that are emitted from the same light source 10, are diffracted at different angles with the rotation of the deflection disc 11, to create a single scanline of beams. The beams diffracted by the deflection disc 11 are reflected by a plurality of reflecting mirrors 13 and 14, so that the traveling direction is changed.
The reflected beams come to pass through a beam correction means. In general, the beam correction means includes a condensing mirror 15 for condensing and reflecting the beam, and an hologram element 16 for diffracting and transmitting the beam to direct the beam toward a photosensitive medium (not shown) such as a photoreceptor web. Alternatively, the beam correction means may be replaced with an F-.theta. lens (not shown) that corrects the focal position and scanwidth of the beam. The F-.theta. lens corrects aberrations of the beam scanned in a primary scan direction and sets the form of the beam as the deflection disc 11 rotates.
Through the above operations, beams emitted from the light source 10 can form a scanline on the photoreceptor web in the primary scan direction, that is, in a direction perpendicular to the traveling direction of the photoreceptor web.
Only one light source 10 is illustrated in FIG. 1. However, a color printer needs a plurality light sources for the colors of yellow A), magenta (M), cyan (C) and black (B). A deflection disc 20 and a plurality of light sources 21, 22, 23 and 24, of a multi-beam scanning system that requires a plurality of light sources, are illustrated in FIG. 2. As the diffraction disc 20 rotates, beams emitted from each of the light sources 21, 22, 23 and 24 diffract and transmit the diffraction patterns formed on each different sector of the deflection disc 20 to create scanlines L1, L2, L3 and L4, respectively. The scan directions of the scan lines L1, L2, L3 and L4 are tangential with respect to the deflection disc 20.
In the multi-beam scanning system, after the beams emitted from the light sources 21, 22, 23 and 24 are diffracted and deflected by the deflection disc 20, the traveling paths of the beams are changed toward the same direction, that is, the X-axis direction, to scan beams parallel onto a photoreceptor web (not shown). For the parallel scanning of the beams, as shown in FIG. 3, there are disposed a plurality of first reflecting mirrors 31, 32 and 33 and a plurality of second reflecting mirrors 41, 42, 43 and 44 over the deflection disc 20. That is, beams emitted from the light sources 21, 22 and 23 are diffracted and deflected while passing through each different sector of the deflection disc 20, are reflected by the first reflecting mirrors 31, 32 and 33, and are then reflected by the second reflecting mirrors 41, 42 and 43, thereby heading in the X-axis direction. Also, the beam emitted from the light source 24 (see FIG. 2), which is diffracted and deflected by the deflection disc 20, is reflected by another first reflecting mirror (not shown) and the second reflecting mirror 44 in sequence, thus heading in the X-axis direction.
Preferably, the second reflecting mirrors 41, 42, 43 and 44 are arranged over the center of the deflection disc 20 at different heights, as shown in FIG. 3, for easy arrangement and scanline stability. However, in the case where the light sources 21, 22, 23 and 24 are symmetrically disposed with respect to the center of the deflection disc 20, as shown in FIGS. 2 and 3, directions of each scanline do not coincide with each other. The problem associated with the symmetrical arrangement of the light sources will be described in greater detail with reference to FIGS. 4A through 4D.
FIG. 4A illustrates the path of beams emitted from the light source 24. That is, a beam emitted from the light source 24 is reflected by the first reflecting mirror 34, and the scanline thereof heads in the X-axis direction. Then, the beam is reflected again by the second reflecting mirror 44 disposed over the center of the deflection disc 20, so that the scanline heads in the -Y-axis direction as indicated by an arrow D1. Similarly, as shown in FIGS. 4B and 4C, scanlines of beams reflected by the first mirrors 32 and 33 and then reflected by the second reflecting mirrors 42 and 43, respectively, which have been emitted from the light sources 22 and 23, also head in the -Y-axis direction as indicated by each arrow D1.
However, referring now to FIG. 4D, the scanline of the beam, which is emitted from the light source 21 and then reflected by the first and second reflecting mirrors 31 and 42 in sequence, heads in the Y-axis direction as indicated by an arrow D2, which is opposite to the scanline directions D1 of the beams emitted from the light sources 22, 23 and 24. Such noncoincidence of the scanline directions must be corrected by an additional circuit or mechanical device prior to scanning it onto a photoreceptor web.
To avoid noncoincidence of the scanline directions, which occurs where the light sources 21) 22, 23 and 24 are symmetrically arranged with respect to the center of the deflection disc 20, a configuration shown in FIG. 5 has been suggested, where all light sources 51, 52, 53 and 54 are arranged within one section divided by a bisecting line S, which passes through the center of a deflection disc 50 and is parallel to the Y-axis.
The light sources 51, 52, 53 and 54 are disposed at an intermediate angle that measures 60.degree.. The arrangement of the light sources 51, 52, 53 and 54, which is illustrated in FIG. 5, provides an advantage of providing the same scanline directions. However, there is a problem associated with asymmetry of scanlines from a light source, which will be described below with reference to FIG. 6.
FIG. 6 shows the path of the beam emitted from the light source 52. The beam emitted from the light source 52 is diffracted and deflected by a predetermined pattern of the rotating deflection disc 50, and is then reflected by a reflecting mirror 55 disposed over the center of the deflection disc 50, so that it heads in the X-direction. Here, a central beam B1 of scanlines travels along a central line C, which is parallel to the X-axis, after being reflected by the reflecting mirror 55. But side beams B2 and B3, which are scanned onto boundaries relative to a central region scanned by the center beam B1, form a predetermined angle with the X-axis. The side beams B2 and B3 are emitted from the light source 52 with the same angle with respect to the central beam B1, however, intermediate angles .theta.1 and .theta.2 of the side beams B2 and B3 with respect to the central beam B1, after being reflected by the reflecting mirror 55, are different. That is, the intermediate angle .theta.1 between the side beam B2 and the central beam B1 is smaller than the intermediate angle .theta.2 between the side beam B3 and the central beam B1. The asymmetry of scanlines, as denoted by the distances T1 and T2 from the central line C, degrades reliability in forming an image and also requires an additional complicated device capable of correcting the asymmetry of scanlines.